1. Field of the Invention
The present invention relates to an apparatus and a method for controlling a fluid pump.
2. Description of the Related Art
There is known a common rail-type fuel injection apparatus wherein a common rail (pressure accumulating chamber) for storing high pressure fuel is provided and a fuel injection valve is connected to the common rail so that fuel is injected into an internal combustion engine.
In the common rail-type fuel injection apparatus, the rate of fuel injection from the fuel injection valve varies in accordance with the common rail pressure, that is, the pressure inside the common rail. Therefore, it is necessary to control the common rail pressure with high precision so that an optimal fuel injection rate can be achieved in accordance with the engine operating conditions.
The common rail pressure is controlled typically by controlling the amount of fuel ejected, i.e., the fuel pumping amount, from a high-pressure fuel supply pump that supplies fuel to the common rail. A plunger-type pump is normally used as the high-pressure fuel supply pump.
In the common rail-type fuel injection apparatus, high pressure fuel stored in the common rail is injected into cylinders from fuel injection valves provided separately for the individual cylinders. Therefore, the pressure in the common rail decreases every time fuel injection is performed. Consequently, there is a need for a fuel pump control apparatus to cause the fuel pump to pump a required amount to the common rail after each fuel injection so as to hold the pressure in the common rail at a target pressure. Moreover, in actual operation, the target common rail pressure itself is sharply varied over a wide range in accordance with the operating condition of the engine during transitional operation, during which the engine operating condition sharply changes. Therefore, during the transitional period, the fuel pump control apparatus needs to control the amount of fuel to be pumped out from the fuel pump, i.e., fuel pumping amount, so as to prevent the pressure in the pressure accumulating chamber from overshooting or undershooting following changes in the target pressure, that is, so as to achieve good controllability of the pressure in the pressure accumulating chamber.
The plunger pump used as the common rail-type fuel pump is normally an inner cam-type plunger pump as shown in FIG. 11. Since the fuel pump needs to pump fuel for the fuel injection into each cylinder of the engine, the number of times of pumping out fuel during one revolution of the pump needs to correspond to the number of cylinders. The pump shown in FIG. 11 has four cam lobes and four plungers. In the pump shown in FIG. 11, the plungers simultaneously pump out and draw in fuel during each cycle, that is, 90.degree. rotation of the pump drive shaft. Therefore, the fuel pump pumps out fuel four times per revolution. In four-stroke engines, the fuel injection into all the cylinders is completed in two engine revolutions. Consequently, the pump shown in FIG. 11 can be used for a four-stroke eight-cylinder engine by driving the pump at the revolution speed equal to that of the crank shaft. The pump can also be used for a four-stroke four-cylinder engine by driving the pump at half the revolution speed of the crank shaft. However, with the four cam lobes of the inner cam as shown in FIG. 11 for driving the plungers, it becomes necessary to set a large changing rate of the cam profile of each cam lobe, which results in greater fluctuation of the pump driving torque. Greater fluctuation of the pump driving torque increases the load on the component parts of the pump driving system, such as the chain or the belt, and therefore may reduce the service life of the pump driving system.
In order to reduce the pump driving torque fluctuation, it is necessary to reduce the number of cam lobes and therefore reduce the changing rate of the cam profile. FIG. 2 shows a two-lobe cam pump in which the number of cam lobes is reduced to two. This cam pump has four plungers, and it is designed so that each oppositely positioned pair of cam lobes simultaneously perform pumping and intake strokes. Each plunger operates at cycles of 180.degree. rotation of the pump drive shaft. With two pairs of plungers, the pump device pumps out fuel four times per rotation of the pump.
As for the method for controlling the amount pumped out of a plunger pump, there are known a pre-stroke adjusting method and an intake adjusting method.
The pre-stroke adjusting method controls the amount pumped from each plunger by holding the intake valve for each plunger at an open position until an intermediate stage of the pumping stroke of the plunger. More specifically, in the pre-stroke adjusting method, each plunger draws an amount of fuel corresponding to the entire stroke of the plunger into the corresponding cylinder during the intake stroke. In an early stage of the pumping stroke, a certain amount of taken-in fuel is discharged from the cylinder through the intake valve. After the intake valve is closed during the pumping stroke, the amount of fuel contained in the cylinder at that time is pressurized by the plunger. When a predetermined fuel pressure is reached, an ejection valve urged by a spring is forced to open, so that fuel is pumped into the common rail.
The intake adjusting method draws a necessary amount of fuel into each cylinder by closing the intake valve for each plunger at an intermediate stage of the intake stroke. Therefore, the entire amount of fuel drawn into each cylinder is ejected from the cylinder during the pumping stroke.
Since the pre-stroke adjusting method closes each intake valve during the pumping stroke, the method needs to employ intake valves designed for use under higher pressures than the intake valves employed by the intake adjusting method. Thus, the cost of the apparatus for the pre-stroke adjusting method becomes comparatively high. Moreover, in the pre-stroke adjusting method, a surplus of the amount of fuel drawn into each cylinder must be discharged from the cylinder by using the corresponding plunger in the early stage of the pumping stroke. Therefore, the pre-stroke adjusting method has a danger of increasing the pump driving power loss, in comparison with the intake adjusting method.
Therefore, it is preferable that the common rail fuel pump be a two-lobe cam pump, which reduces the driving torque fluctuation, and the amount of fuel to be pumped out of the cam pump be controlled by the intake adjusting method, which reduces the apparatus cost and the power loss.
However, the combination of a two-lobe cam pump and the intake adjusting method conventionally causes the problem of deterioration of responsiveness in the common rail pressure control.
Whereas the pre-stroke adjusting method determines the amount of fuel to be pumped from each plunger on the basis of the intake valve closing timing during the pumping stroke of the plunger, the intake adjusting method determines the amount of fuel to be pumped from each plunger on the basis of the intake valve closing timing, i.e., the intake valve open period, during the intake stroke of the plunger. Therefore, the pre-stroke adjusting method allows control of the pumping amount in accordance with the engine operating condition and the common rail pressure immediately before the start of pumping, that is, immediately before the start of closing the intake valve. On the other hand, the intake adjusting method necessitates determining the pumping amount in an early stage of the intake stroke. Therefore, in the intake adjusting method, a time interval between the determination of the pumping amount and the actual start of pumping becomes long. If, during the time interval, the engine operating condition or the common rail pressure changes, such a change may not be able to be reflected in the pumping amount.
This problem with the intake adjusting method becomes more significant if the method is applied to a two-lobe cam pump. With reference to FIG. 12, problems with a common rail-type fuel injection apparatus for a four-stroke four-cylinder engine employing a two-lobe cam pump controlled by the intake adjusting method will be described below.
In the chart of FIG. 12, line (A) indicates changes in the common rail pressure. The common rail pressure decreases in accordance with the amount of fuel injected, at every fuel injection into each cylinder. Subsequently, the common rail pressure is increased by the fuel pump pumping fuel to the common rail. In FIG. 12, points indicated by #1, #3, #4 indicate pressure drops due to three consecutive fuel injecting operations for first, third and fourth cylinders, respectively. Vertical lines T.sub.1, T.sub.2, T.sub.3 indicate time points of setting amounts of fuel to be pumped from the fuel pump, where the interval between T.sub.1, and T.sub.2 and the interval between T.sub.2 and T.sub.3 are 180.degree. in terms of crank shaft revolution angle. Line (B) indicates the target pressure PCTRG in the common rail. The target common rail pressure is set in accordance with the engine operating condition, at the time of setting an amount of fuel to be pumped.
According to a typical conventional fuel pump control, the fuel pumping amount is determined as the sum of a feed forward amount that is determined by a fuel injection amount instruction value and the common rail pressure at the time of setting a pumping amount, and a feedback amount that is determined by the difference between the target common rail pressure and the actual common rail pressure at the time of setting the pumping amount.
Lines (C) in FIG. 12 indicate stroke cycles of two pairs of plungers of an intake adjusting-type two-lobe cam pump. Since the two-lobe cam pump for a four-stroke four-cylinder engine is rotated at half the speed of that of the engine crank shaft, the two pairs of plungers (plunger group A and plunger group B) alternately pump out fuel at every 180.degree. of crank shaft rotational angle.
Line (D) in FIG. 12 indicates stroke cycles of a pre-stroke adjusting-type four-lobe cam pump. The four-lobe cam pump is driven at half the revolution speed of the crank shaft, so that the four-lobe cam pump pumps out fuel at every 180.degree. crank revolution.
As indicated by line (D) in FIG. 12, the four-lobe cam pump completes one stroke cycle of pumping and intake strokes at every 180.degree. crank angle revolution. The pumping amount is determined by the intake valve closing timing during the pumping stroke. Therefore, the amount of fuel calculated at time point T.sub.1 in FIG. 12 is completely pumped out at time point P.sub.1 indicated on line (D). The amount of fuel to be pumped out is set in accordance with the common rail pressure at time point T.sub.1 and the fuel injection amount instruction value at that time point (that is, the amount of fuel to be injected into the first cylinder), and the difference between the target pressure PCTRG and the actual pressure PC.sub.1 at time point T.sub.1, as stated above. Therefore, when the pumping of fuel is completed at time point P.sub.1, the common rail has been supplied with an amount of fuel that completely compensates for the common rail pressure fall due to the fuel injection into the first cylinder and the deviation of the actual common rail pressure from the target pressure occurring at time point T.sub.1. Consequently, at time point P.sub.1, the actual common rail pressure becomes precisely equal to the target pressure PCTRG.
In the intake adjusting-type two-lobe cam pump, the stroke cycle of each plunger is 180.degree. as indicated by line (C). The pumping fuel amount set at time point T.sub.1 is taken in by the intake stroke of the plunger group A, and supplied to the common rail at time point P.sub.1 ' indicated on line (C), which follows the end of fuel injection into the third cylinder after the fuel injection into the first cylinder. Thus, the pumping fuel amount set on the basis of the conditions occurring at time point T.sub.1 has not been supplied to the common rail before the next time point (T.sub.2) for setting an amount of fuel to be pumped. More specifically, the timing of the effect of the pumping amount setting is delayed 180.degree., compared with the timing in the four-lobe cam pump.
Moreover, in the case of the two-lobe cam pump, the fuel pumping by the plunger group B occurs during the period between the pumping amount setting time point T.sub.1 for the plunger group A and the time point P'.sub.1 of completion of actual fuel supply from the plunger group A. Therefore, the actual common rail pressure at the time of completion of fuel pumping from the plunger group A differs from the common rail pressure at time point T.sub.1. Consequently, if the conventional feed forward/feedback control is performed using the intake adjusting-type two-lobe cam pump, the controllability of the common rail pressure at the time of a change of the target fuel pressure deteriorates so that the common rail pressure becomes likely to overshoot or undershoot.
This problem will be described with reference to FIG. 14.
The diagram of FIG. 14 indicates changes in the target and actual common rail pressure where the feed forward control and the feedback control based on the deviation of the actual common rail pressure from the target pressure is performed using an intake adjusting-type two-lobe cam pump, according to the conventional art. In FIG. 14, t.sub.0 through t.sub.8 indicate the timing of pumping fuel from the fuel pump; PCTRG indicates a change in the target common rail pressure, i.e., an instruction value; and PC indicates changes in the common rail pressure occurring if the amount of fuel pumped from the fuel pump is controlled by the conventional feed forward/feedback control. In FIG. 14, it is assumed that the target common rail pressure PCTRG is greatly changed from PCTRG.sub.0 to PCTRG.sub.1, and that the target value PCTRG remains constant and equal to the common rail pressure up to t.sub.0.
If the target common rail pressure is changed at time point t.sub.1, the feedback amount TFBK is set in accordance with the difference .DELTA.P.sub.0 between the changed target pressure PCTRG.sub.1 and the actual common rail pressure PCTRG.sub.0. On the other hand, the feed forward amount TFBSE is set in accordance with the changed target pressure. If the target pressure is not changed, the value of the feed forward amount TFBSE is maintained. If the target pressure is changed at time point T.sub.1, the pumping amount from the fuel pump is changed in accordance with the change in the target pressure. However, since the target pressure change is actually large, the set fuel pumping amount considerably exceeds a predetermined maximum fuel pumping amount Q.sub.MAX, that is, the entire amount of fuel required cannot be supplied by one fuel pumping operation. Since the fuel pumping operation must be performed a plurality of times to supply the required amount of fuel, the actual common rail pressure is increased stepwise after the target pressure is changed. Although the actual pressure increasing pattern is different from the pressure increasing pattern indicated in FIG. 14 since fuel injection is performed during the fuel pumping operation, the common rail pressure fluctuation due to fuel injection is ignored in the diagram of FIG. 14 to simplify the illustration.
In the intake adjusting-type two-lobe cam pump, the time point of setting a fuel pumping amount and the time point of actually pumping out fuel from a plunger group are interposed by the pumping of fuel from the other plunger group. If the common rail pressure is increased stepwise as indicated in FIG. 14, the amount of fuel set on the basis of, for example, the pressure difference .DELTA.P.sub.3 at time point t.sub.3, is actually pumped out of a plunger group at time point t.sub.5, and the fuel pumping from the other plunger group is performed at the intervening time point t.sub.4. As a result, the common rail pressure occurring at time point t.sub.5 becomes higher than that occurring at the fuel pumping amount setting time point (t.sub.3). More specifically, the amount of fuel supplied to the common rail by the fuel pumping operation performed at time point t.sub.5 corresponds to the pressure difference .DELTA.P.sub.3 occurring at time point t.sub.3 in FIG. 14, which is considerably greater than the pressure difference .DELTA.P.sub.4 occurring immediately before the actual fuel pumping operation at time point t.sub.5. Therefore, the operation of setting a pumping amount at time point t.sub.3 and pumping the set amount of fuel at time point t.sub.5 causes the common rail pressure to exceed the target pressure, that is, causes an overshoot. In fact, at the next fuel pumping (t.sub.6), the actual common rail pressure exceeds the target pressure, so that the fuel pumping amount must be reduced. Nevertheless, at time point t.sub.6, the amount of fuel set on the basis of the pressure difference .DELTA.P.sub.4 at time point t.sub.4 is pumped out, so that the common rail pressure further increases, that is, overshoots. Since there is a difference between the common rail pressure at the time of setting a fuel pumping amount and the common rail pressure at the time of actually pumping the set amount of fuel, an overshoot of the common rail pressure is followed by an undershoot (t.sub.8) at the time of the next or later fuel pumping operation. Furthermore, the common rail pressure may hunt, so that the controllability of the common rail fuel pressure may deteriorate. Although deterioration of the controllability can be reduced to some extent by changing the gain in the feedback control in accordance with the engine operating condition as in the related-art apparatus, it is still difficult to sufficiently reduce or prevent the aforementioned overshoot or undershoot according to the related art.
Deterioration of the controllability of the common rail pressure, especially, overshoot of the common rail pressure, is unfavorable because such an event is likely to lead to an increase of engine noise and deterioration of emissions control.
Although the problems of the related art have been described with regard to the case where an intake adjusting-type two-lobe cam pump is used for the common rail in a four-cylinder engine, similar problems may also occur in engines having other numbers of cylinders. That is, if an intake adjusting-type two-lobe cam pump is used in a common rail-type fuel injection apparatus in an engine, the problems of deterioration of controllability of the common rail pressure may occur at the time of transitional operation of the engine.